Lasing in the range of near-infrared to terahertz waves is difficult in recombination emission of holes and electrons.
A cascade connection of multi-quantum well structures enables lasing in the range of infrared to terahertz waves (30 GHz to 30 THz) by inter-subband transition of electrons.
In this case, in a Fabry-Perot laser with mirrors formed at the device edge surfaces, the thickness of the edge emission surface is small, and the optical density is high. This limits the increase of output power.